Running that far, that fast, on pavement with no protective shoeing means the horse likely has what is called mechanical founder -- their hooves literally separate from the tissue underneath and the bones of the feet can come through the bottom. It's a horribly painful thing; there is no medical correction if it goes catastrophic (and even a severe case leaves the animal crippled and in pain the rest of its life), so the best thing to do is to euthanize it.

Horses can't lay on their sides for long periods of time; their body weight is too much, and they need to be able to move around in order to keep blood flowing to their extremities and to help digest their food.